This invention relates to a method and apparatus for controlling the speed of a slave drive roller conveying a web by adjusting the range of allowable speed variability of the slave drive roller to different operating conditions. More particularly, the invention relates to such a method and apparatus used for conveying a substrate for photographic film or paper and coating it with photosensitive coatings.
In conveying a web through a coating machine, web speed influences how thick a coating will be applied at a coating station. That is, given a constant flow rate of coating fluid at a coating station, the higher the web speed the thinner the coating on the web. Likewise, the lower the web speed, the thicker the coating. Thus, coating thickness is an inverse function of web speed. If coating thickness uniformity is critical, then web speed uniformity at the coating station is also critical.
In conveying and coating a web for photographic applications, coating thickness is indeed critical. Hence, web speed must be closely controlled.
In a typical method of web conveyance, one drive roller is selected as the master drive roller; the speed of any other drive roller is slaved or controlled with reference to the master drive signal. The controlled speed of the slave drive roller nevertheless varies, within a certain tolerance, from the actual speed of the master drive roller to maintain operating tension in that portion of the machine. Thus, the rotational speed of the slave drive roller is increased or decreased depending on how much web material is stored in a float roller controlling the slave drive roller. Such typical control methods and equipment are described in, for example, W. Boice, "Controlling Speed in Multidrive Systems", in Machine Design Jan. 22, 1970; D. Satas, "Web Handling" in Web Processing and Converting Technology and Equipment, 1984; H. Weiss, "Tension Transducers" pages 78-97 in Control Systems for Web-Fed Machines, 1983; and Machine Drive Systems pages 136-143.
In considering certain requirements for a proposed new product, it was appreciated that conventional methods of controlling slave drive roller speed had disadvantages. For example, in applications requiring narrow tolerances between the speed of the master roller and speed of the slave roller, this narrow tolerance is disadvantageous during periods of acceleration or deceleration, for example, during startup or shutdown of the web conveyance operations. This is because excessive time is consumed in acceleration and deceleration of the web when the slave roller is controlled with a small tolerance.
Accordingly, it is one object of the invention to provide a method and apparatus for controlling the speed of the slave drive roller within a small tolerance at times when coating operations are being performed, without sacrificing excessive time during times of acceleration up to running speed and deceleration from running speed. In other words, it is an object of the present invention to provide high gain during times when the coating machine is unstable, to avoid slack or tight web conditions; and to provide low gain during times of normal steady run conditions, to minimize speed deviations at any critical secondary follower or slave drive, such as a second coater in a coating machine; and to change automatically. A higher gain indicates that the control system adjusts the speed of the slave drive roller more quickly and/or by a greater amount in response to changes in control inputs.
It is further an object of the invention to provide a method and apparatus wherein gain is a function of acceleration/deceleration rate such that low gain is used at steady run speed, and higher gain is used during acceleration/deceleration (for example, where gain is proportional to acceleration/deceleration rate).
It is further an object of the invention to provide a method and apparatus wherein gain is a function of float roller position (that is, deviation from a predetermined normal position) such that low gain is used at or near the normal position, and higher gain is used when the float roller is displaced from the normal position (for example, gain may be proportional to deviation, or gain may be proportional to the square (or some other power) of deviation).
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be evident from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.